Ti-15Mo-3Al-2.7Nb-0.2Si钛合金绝热剪切带的微观结构演化

采用分离式Hopkinson压杆技术对Ti-15Mo-3Al-2.7Nb-0.2Si钛合金的帽形试样进行了强迫剪切试验,通过扫描电镜(SEM)及透射电镜(TEM)研究了Ti-15Mo-3Al-2.7Nb-0.2Si钛合金在动态加载下绝热剪切带的微观结构演化.结果表明:Ti-15Mo-3Al-2.7Nb-0.2Si钛合金由于其组织以bee晶格的a相为主,具有较好的变形能力,因此其绝热剪切带的形成是位错运动的结果;剪切带的微观结构演化过程为:晶粒在外加切应力作用下拉长变形一拉长晶粒的破碎-形成呈一定方向排列的细小等轴晶:带内形成的细小等轴晶尺寸为O.2～0.4ìm.     

Microstructural control of Ti-Al-Mo-Nb alloy system with respect to variation of B

In the current study, phase stability of Ti-Al-Mo-Nb alloys was investigated, and the effect of B addition was examined for cast alloys. The fabricated cast alloys were mainly composed of α₂ / γ lamellar with a β phase, when they were heat treated at 1100 °C followed by air cooling, the alloy was composed of α2 / γ lamellar with γ+β necklace phase at the colony boundary for the Ti-45Al-3Mo-2Nb-1B alloy, and the colony size was refined to ~ 20 μm. In order to identify the effect of the microstructures on mechanical strength, compressive tests were performed on the fabricated alloys of Ti-45Al-3Mo-2Nb and Ti-45Al-3Mo-2Nb-1B at room temperature and at 800 °C. The microstructural variations and phase stability were discussed in terms of pseudo-binary phase diagram calculated by Pandat software?.     

INFLUENCE OF ALLOYING ELEMENTS（Nb，Mo，V） ON MICROSTRUCTURE OF Ti_3Al BASE ALLOYS

ＩＮＦＬＵＥＮＣＥＯＦＡＬＬＯＹＩＮＧＥＬＥＭＥＮＴＳ（Ｎｂ,Ｍｏ,Ｖ）ＯＮＭＩＣＲＯＳＴＲＵＣＴＵＲＥＯＦＴｉ_３ＡｌＢＡＳＥＡＬＬＯＹＳ￥ＳＯＮＧＤａｎ;ＤＩＮＧＪｉｎｊｕｎ;ＷＡＮＧＹａｎｄｏｎｇ（ＡｎａｌｙｓｉｓａｎｄＴｅｓｔｉｎｇＣｅｎｔｅ?..     

Effect of carbon on microstructures of Ti-45Al-3Fe-2Mo-xC alloy

The effect of carbon addition on the microstructures of TiAl-based alloy (Ti-45Al-3Fe-2Mo) was studied. The proportion of β/B2 phase reduces with the content of carbon increasing, while the colony size increases. With increasing the carbon content, the lamellar spacing first decreases from 267 nm (Ti-45Al-3Fe-2Mo) to 237 nm (Ti-45Al-3Fe-2Mo-0.3C) and 155 nm (Ti-45Al-3Fe-2Mo-0.5C), but then increases to 230 nm (Ti-45Al-3Fe-2Mo-1.0C) with further increase in C level, which is affected by the inhibition of carbon atom and precipitation of carbides at the lamellar interface. Precipitation of carbides shows a response to aging time at 800 °C. P-type carbides grow up at the boundaries and near the dislocation areas with the prolonging of aging time. And these carbides are projected different morphology in different beam directions (BD). The effects of these microstructural modifications were examined and the observations were discussed.     

The Solidification Mode of Fe-Mn-Al-C Lightweight Steel

Solidification behavior and solid-state transformation were investigated in Fe-Mn-Al-C lightweight steel. The solidification mode of the Fe-9.3Mn-5.6Al-0.2C (wt.%) lightweight steel was predicted to be the F mode (liquid → liquid + δ-ferrite → δ-ferrite → δ-ferrite + γ-austenite), according to the classification of the solidification modes of stainless steels. However, the microstructures of an ingot of the lightweight steel showed that the solidification occurred by the FA mode (liquid → liquid + δ-ferrite → liquid + δ-ferrite + γ-austenite → δ-ferrite + γ-austenite). To examine the difference between predicted and actual solidification modes, some specimens were annealed at various temperatures ranging from 1200°C to 1450°C for 10 min and then quenched. The microstructures of the annealed specimens exhibited that there were the narrow ranges of single δ-ferrite and solidification sections. This result indicates that the solidification mode of the present steel was the F mode, matching well with the predicted solidification mode. The reason for the FA mode, which was observed in the ingot, was because the liquid passed by a narrow solidification section and a single δ-ferrite region, and it directly entered the dual-phase region of δ-ferrite and γ-austenite during cooling.     

Ta含量对Ti-6Al-3Nb-2Zr-1Mo-xTa合金力学性能和腐蚀性能的影响

系统研究了Ti-6Al-3Nb-2Zr-1Mo-x Ta(x=0,0.2,0.5,1.0,3.0,5.0)合金的微观组织、拉伸性能、夏比冲击韧性和耐海水腐蚀性。结果表明,经α+β两相区锻造后,Ti-6Al-3Nb-2Zr-1Mo-5Ta合金获得片层组织,Ti-6Al-3Nb-2Zr-1Mo-x Ta(x=0,0.2,0.5,1.0,3.0)均获得双态组织。XRD、TEM和选区电子衍射表明,在添加Ta元素后,Ti-6Al-3Nb-2Zr-1Mo-x Ta合金没有新相产生。对于双态组织Ti-6Al-3Nb-Zr-1M0-x Ta合金,随着Ta含量的增加,其Mo当量逐渐增加,导致其屈服强度、抗拉强度和显微硬度均有所提高。而Ta含量对冲击吸收功的影响规律与屈服强度和抗拉强度的影响规律相反,其大小与冲击断口剪切唇区面积一致。当Ta含量超过1.0%(质量分数)时,由于α和β相之间的标准平衡电位差逐渐增大,Ti-6Al-3Nb-2Zr-1Mo-x Ta合金的耐海水腐蚀逐渐降低。综合考虑强度、冲击韧性和耐海水腐蚀性能,Ti-6Al-3Nb-2Zr-1Mo-1Ta合金综合匹配性最好,具有良好的海洋工程应用潜力。     

Enhancement of creep properties and microstructural stability of intermetallic β-solidifying γ-TiAl based alloys

Carbon, Mo and Si represent promising alloying elements with respect to increase the operating temperature of intermetallic titanium aluminides. The influence of these elements on microstructural stability and creep properties is investigated on a β-solidifying γ-TiAl based alloy, named TNM, with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%) in two different microstructural conditions. The first condition after casting and hot isostatic pressing has a coarse “nearly lamellar γ” microstructure. The second condition is adjusted by a subsequent heat treatment and shows a “nearly lamellar β” microstructure with finely spaced α₂ and γ lamellae and areas of discontinuous precipitation (cellular reaction) in the α₂/γ-colonies. Creep tests were carried out at 815 °C and 150 MPa to examine the influence of microstructure and its change on and during creep. Alloying of C and Si or Mo to the TNM based material led to improved creep properties and microstructural stability by inhibiting the progress of discontinuous precipitation.     

Experimental Study of Phase Equilibria in the System Cu-Al-Sn

Phase equilibria in the Cu-rich corner of the ternary system Cu-Al-Sn have been re-investigated. Final equilibrium microstructures of 20 ternary alloy compositions near Cu₃Al were used to refine the ternary phase diagram. The microstructures were characterized using optical microscopy (OM), x-ray diffraction (XRD), electron probe microanalysis and transmission electron microscopy. Isothermal sections at 853, 845, 833, 818, 808, 803 and 773 K have been composed. Vertical sections have been drawn at 2 and 3 at% Sn, showing β₁ as a stable phase. Three-phase fields (α + β + β₁) and (β + β₁ + γ₁) result from β → α + β₁ eutectoid and β + γ₁ → β₁ peritectoid reactions forming metastable β₁ in the binary Cu-Al. With the lowering of temperature from 853 to 818 K, these three-phase fields are shifted to lower Sn concentrations, with simultaneous shrinkage and shifting of (β + β₁) two-phase field. The three-phase field (α + β + γ₁) resulting from the binary reaction β → α + γ₁ shifts to higher Sn contents, with associated shrinkage of the β field, with decreasing temperature. With further reduction of temperature, a new ternary invariant reaction β + β₁ → α + γ₁ is observed at ~813 K. The β disappears completely at 803 K, giving rise to the three-phase field (α + β₁ + γ₁). Some general guidelines on the role of ternary additions (M) on the stability of the ordered β₁ phase are obtained by comparing the results of this study with data in the literature on other systems in the systems group Cu-Al-M.     

Microstructure and mechanical properties of low- and heavy-alloyed intermetallic alloys based on γ-TiAl + α2-Ti3Al

The microstructure and mechanical properties of the Ti-43.7Al-3.2(Nb,Cr,Mo)-0.2B alloy in the as-cast state (after gasostatistic processing) and of the Ti-45Al-8Nb-0.2C alloy after hot extrusion at temperatures corresponding to the ?? + ?? phase field followed by heat treatment have been studied. The extruded heavy-alloyed alloy has demonstrated significantly higher plastic/mechanical properties at room temperature with close values of the plasticity/tensile strength and long-term strength at elevated temperatures. A comparison of the results with literature data has shown the properties of the as-cast Ti-43.7Al-3.2(Nb,Cr,Mo)-0.2B to be similar to or superior to those of the best-known casting ?? (TiAl) + ??₂ (Ti₃Al) alloys.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

Effects of minor yttrium addition on hot deformability of lamellar Ti-45Al-SNb alloy

The effects of 0.3%（molar fraction, the same below） yttrium addition on hot deformability of lamellar Ti-45Al-5Nb alloy were investigated by simulated isothermal forging tests. The ingots with the nominal compositions of Ti-45Al-5Nb and Ti-45Al-5Nb-0.3Y were prepared by induction skull melting. Simulated isothermal forging tests were conducted on Gleeble 1500D thermo-simulation machine using a 6 mm in diameter and 10 mm in length compressive specimen at the deformation temperatures of 1 100, 1 150, 1 200 ℃ and strain rates of 1.0, 0.1, 0.01 s^-1. The results show that yttrium addition remarkably improves hot deformability of Ti-45Al-5Nb alloy. An appropriate hot deformation processing parameter of Ti-45Al-5Nb-0.3Y alloy is determined as 1 200 ℃, 0.01 s^-1. The flow stresses are decreased by yttrium addition under the same compressive conditions. The activation energies of deformation Q are calculated as 448.6 and 399.5 kJ/mol for Y-free and Y-containing alloys, respectively. The deformed microstructure observation under 1 200 ℃, 0.01 s^-1 condition indicates that Ti-45Al-5Nb-0.3Y alloy shows more dynamic recrystallization. The improvement of hot deformability of Ti-45Al-5Nb-0.3Y alloy induced by yttrium addition should be attributed to that the smaller the original lamellar colonies, the lower the deformation resistance and activation energy of deformation are, and the more the dynamic recrystallization is.     

Effect of Al-5Ti-1B-1Re on the Microstructure and Hot Crack of As-Cast Al-Zn-Mg-Cu Alloy

High crack tendency is easy to occur during preparation of the castings and ingots of Al-Zn-Mg-Cu alloy, and it is most difficult to be solved due to the characteristics for the series alloy. As-cast microstructures and hot cracking tendency of Al-Zn-Mg-Cu alloy with different addition levels of Al-5Ti-1B-1Re were investigated in the paper. Moreover, solidification characteristics of the experimental alloys with different mass fractions of Al-5Ti-1B-1Re were analyzed, and the addition content of Al-5Ti-1B-1Re was optimized. These results indicate that the microstructure of the experimental alloys with Al-5Ti-1B-1Re refiner is fine obviously, the dendrite shape refined by Al-5Ti-1B-1Re becomes more globular, the grain boundary is smoother, and the SDAS is smaller compared with the alloy without Al-5Ti-1B-1Re. When the addition level of the grain refiner is less than 0.2%, the hot cracking tendency for the experimental alloy is reduced. The addition of 0.2% Al-5Ti-1B-1Re is best effective to improve the as-cast microstructures and hot cracking of Al-Zn-Mg-Cu alloy, and the best addition level of Al-5Ti-1B-1Re refiner is 0.2%.     

Grain Refining Efficiency of SHS Al-Ti-B-C Master Alloy for Pure Aluminum and Its Effect on Mechanical Properties

A new Al-5Ti-0.75B-0.2C master alloy was successfully prepared by self-propagating high-temperature(SHS)reaction from an Al-Ti-B_4C system with molten Al.Microstructure and phase characterization of the prepared Al-5Ti-0.75B-0.2C master alloy show that the nearly spherical TiC particles,hexagonal or rectangular TiB_2 particles,and blocklike TiAl_3 particles distribute uniformly in the aluminum matrix.Grain refining test on commercial pure aluminum indicates that Al-5Ti-0.75B-0.2C master alloy exhibits a better grain refining performance than Al-5Ti-lB master alloy.By addition of 0.2 wt%Al-5Ti-0.75B-0.2C master alloy,the average grain size of a-Al can be effectively refined to160 ± 5 μm from about 3000 μm,and the tensile strength and elongation are increased by about 20%and 14.1%due to the grain refinement.     

Effects of Pack Rolling Temperature on Microstructure and Mechanical Properties of Powder Metallurgical Ti-45Al-7Nb-0.3W Alloy

Powder metallurgical Ti-45Al-7Nb-0.3W (at.%) alloys were pack rolled at temperatures of 1240°C, 1255°C, 1270°C, and 1285°C. The microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The tensile properties were tested at room temperature and 800°C. After rolling, the sheets exhibited duplex microstructures with refined grains. The tensile test results showed the sheet rolled at 1270°C displayed excellent room temperature tensile properties with an ultimate tensile strength (UTS) of 782 MPa and an elongation of 1.95%. When tested at 800°C, all sheets showed UTS of over 600 MPa and elongations of around 50%. The dislocation movements and mechanical twinning played important roles at the initial stage of rolling deformation. However, during the subsequent deformation process, the deformation mechanism should mainly be the result of dynamic recrystallization.     

Effects of cooling rate and initial composition on the solidification path and microstructure of Al-Cu-Si alloys

The influences of cooling rate and initial composition on solidification path, microstructure and hardness of Al-Cu-Si alloys were investigated. The results indicate that the solidification paths of alloys Al-3.7Cu-6.5Si, Al-5.2Cu-2.6Si and Al-15.1Cu-6.2Si were (L + α-Al)→(L + α-Al+ β-Si)→(L + α-Al+ β-Si+ θ-Al₂Cu), and the solidification paths of alloys Al-4.6Cu-0.9Si, Al-24.2Cu-3.9Si and Al-26.9Cu-2.1Si were (L + α-Al)→(L + α-Al+ θ-Al₂Cu)→(L + α-Al+ β-Si+ θ-Al₂Cu). Furthermore, it is found that the further the initial compositions from the binary eutectic trough and the slower the cooling rate was, the second dendrite arm spacing was larger and the volume percent of phase θ-Al₂Cu was fewer, and then influence the hardness. There were three kinds of (α-Al+ θ-Al₂Cu) binary eutectic, two kinds of (α-Al+ β-Si+ θ-Al₂Cu) ternary eutectic and one kind of (α-Al+ β-Si) binary eutectic morphologies existed in the solidification specimens.     

A study on microstructure development and oxidation phenomenon of arc consolidated Mo-Nb-Si-(Y) alloys

Two alloys of compositions Mo-26Nb-19Si and Mo-26Nb-19Si-0.5Y (at.%) were prepared by non-consumable arc melting. The alloys characterized using BSE, EDS and EBSD techniques confirmed the formation of two phase (Mo, Nb)_SS-(Mo, Nb)₅Si₃ microstructures. Y-rich (Y₂O₃) particles were mainly found to be present at the inter-phase boundaries creating different phase morphologies in Mo-26Nb-19Si-0.5Y alloy. Both the phases showed a preferred orientation indicating a directional growth during solidification. Oxidation tests conducted in air at 1000 and 1300 °C showed a porous oxide scale formation on the alloy surface and loss of material due to evaporation of MoO₃. The detailed SEM-EDS analysis of the oxide scale formed on these alloys in argon environment at 1000 °C, indicated the presence of Nb₂O₅ particles in the silica (SiO₂) matrix.     

Microstructure and fracture toughness of a β phase containing TiAl alloy

Microstructures and fracture toughness of Ti-45Al-2Nb-1.5V-1Mo-0.3Y alloy have been investigated. The alloy exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with mixtures of γ and residual β phases along lamellar colony boundaries. Precipitation of both β and γ phases from α₂ lamellae was found after aging at 950 °C for 48 h. Phase transformations involving β phase both in α₂ laths and along colony boundaries are discussed. This TiAl alloy possesses a higher K_IC value up to 23.5 MPam^1/2 at room temperature, compared with fully lamellar Ti-45Al-5Nb-0.3Y alloy. The toughening mechanism for current alloy is concluded as trans-lamellar fracture, ligament bridges and crack deflection, together with precipitation of β and γ phases. The precipitation of fine β and γ particles is considered as intrinsic toughening mechanism, because α₂/β and α₂/γ interfaces generating due to precipitation can restrict dislocation motion effectively.     

Effect of Hypoeutectic Boron Additions on the Grain Size and Mechanical Properties of Ti-6Al-4V Manufactured with Powder Bed Electron Beam Additive Manufacturing

In additive manufacturing, microstructural control is feasible via processing parameter alteration. However, the window for parameter variation for certain materials, such as Ti-6Al-4V, is limited, and alternative methods must be employed to customize microstructures. Grain refinement and homogenization in cast titanium alloys has been demonstrated through the addition of hypoeutectic concentrations of boron. This work explores the influence of 0.00 wt.%, 0.25 wt.%, 0.50 wt.%, and 1.0 wt.% boron additions on the microstructure and bulk mechanical properties of Ti-6Al-4V samples fabricated in an Arcam A2 electron beam melting (EBM) system with commercial processing parameters for Ti-6Al-4V. Analyses of EBM fabricated Ti-6Al-4V + B indicate that the addition of 0.25–1.0 wt.% boron progressively refines the grain structure, and it improves hardness and elastic modulus. Despite a reduction in size, the β grain structure remained columnar as a result of directional heat transfer during EBM fabrication.     

Isothermal compression behavior and constitutive modeling of Ti-5Al-5Mo-5V-1Cr-1Fe alloy

Hot compression behavior of Ti-5Al-5Mo-5V-1Cr-1Fe alloy with an equiaxed (α+β) starting microstructure was investigated by isothermal compression test and optical microscopy. Based on the true strain–stress data with temperature correction, constitutive models with a high accuracy were developed and processing maps were established. Strain inhomogeneity at different locations in the compressed sample is reduced by raising temperature, leading to a uniform distribution of α phases. For the temperature range of 800–840 °C with a strain rate of 10 s^?1, the transformed volume fraction of α phase increases and the average grain size of α phase decreases slightly with increasing the temperature, indicating co-existence of dynamic recovery and dynamic recrystallization. Flow localization and faint β grain boundaries are observed at the strain rate of 10 s^?1 in the temperature range of 860–900 °C. The processing map analysis shows that hot working of Ti-5Al-5Mo-5V-1Cr-1Fe alloy should be conducted with the strain rate lower than 0.01 s^?1 to extend its workability.     

Effect of enamel coating on long-term oxidation and hot corrosion behavior of Ti-24Al-17Nb-0.5Mo alloys

The effect of enamel coating on long-term isothermal oxidation at 700 ℃ and cyclic oxidation at 800 ℃ in air and hot corrosion resistance of Ti-24Al-17Nb-0. 5Mo in 75% (Na2SO4 K2SO4 ) 25% NaCl (mass fraction) molten mixed salts at 700 ℃ was investigated. The results indicate that Ti-24Al-17Nb-0.5Mo alloy exhibits relatively poor long-term oxidation resistance due to the formation of Al2O3 TiO2 Nb2O5 mixed scales and poor hot corrosion resistance due to the spallation of scales formed in molten (Ns, K)2 SO4 NaCl. Enamel coating can effectively protect Ti-24Al-17Nb-0.5Mo alloy from long-term oxidation at high temperature in air and remarkably improve the hot corrosion resistance of Ti-24Al-17Nb-0. 5Mo alloy, and can act as the barrier to suppress the migration of oxygen and corrosive ions into the substrate.